Halos galore!

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huntsville optics
Mike Moss

Most of us have looked up and caught a view of the occasional halo or sun dog, perhaps even a brightly colored circumzenithal or circumhorizon arc more infrequently. Last week a patch of thin cirrus cloud cover over Huntsville, AL caught a lot of people's attention for the plethora of optical phenomena it produced all at the same time. Luckily, a solar physicist from Marshall Space Flight Center named David Hathaway got outside and captured a really nice wide-angle image showing the array of arcs, halos and circles being produced by what had to be a near-perfect combination of a variety of ice crystals that were very well-formed and perfectly oriented to put on such a show.

The photo has been making the rounds of the internet, but I thought I'd post it here for anyone who may have missed it. Visible in the single image is (at least) a 22-degree halo, parhelia (also called "sun dogs"), a parhelic circle, a tangent arc, a Parry arc, a supralateral arc, a circumzenithal arc and a heliac arc. Each of these is associated with ice crystals having various shapes and orientations, and to have a layer of cirrus clouds that is both thin enough for sunlight to readily pass through but also sufficiently stocked with such a range of well-formed crystals is quite rare. I've included links to the Atmospheric Optics site, where you can find much more information about how these displays, and many others, are generated.

In this case, as the second image shows, the upper level moisture was part of the overall flow surrounding the upper low and circulation associated with former hurricane Sandy, at the time located over the northeastern United States. Most areas farther north and east would have had clouds too thick to allow any such displays, but the thin, patchy high clouds on the southwest fringe obviously ended up "just right" for the photographer in northern Alabama!

Speaking of the photographer, it's also been reported that he used a technique called "high dynamic range" photography for the image, in which several exposures are taken and then combined in such a way that both very bright and rather faint features can be clearly seen. This turns out to be a pretty interesting topic in itself - there is a Wikipedia article on that subject that includes several really interesting examples of the component photos and final products.

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